Gas discharge lamps are extremely popular for providing lighting. For example, they are used in offices, homes, factories, auditoriums, and airliners.
One of the most functional types of gas discharge lamps is inductively powered as described in U.S. Pat. No. 6,731,071, entitled “Inductively Powered Lamp Assembly.” This lamp includes a coil within the lamp envelope for powering each filament or electrode. Each coil is inductively coupled to a power source within the fixture. Optionally, the lamp filaments are provided with a preheat circuit to preheat the filaments before the lamp is started. The circuit includes a switch that is closed to provide preheat current to the filament. After the lamp filament is heated sufficiently, the switch is opened to provide voltage for striking the lamp.
In lamps that are not inductively powered (i.e. that include conventional contact pins extending from the lamp envelope), heating of the lamp filaments is common. Heating of the filaments reduces the voltage required to strike the lamp and to maintain the illumination of the lamp. Additionally, heating of the lamp filaments allows for increased control of dimmability of the lamp. Changing the intensity of a fluorescent lamp requires changing the voltage applied to the lamp. However, reduction in the voltage applied to a lamp reduces the current passing through the filaments of the lamp, thereby changing the temperature of the lamp filaments. If the filament temperature falls too low, the lamp will extinguish because of an inability to maintain the arc between the filaments. Accordingly, ballast circuits have been developed for dimming fluorescent lamps by increasing the current through the filaments as the voltage to the lamp is decreased. These circuits enable the lamp to be dimmed over a greater range. Unfortunately, this approach is not directly adaptable to inductively powered lamps.
An inductively powered gas discharge lamp having an ability to provide filament heating is desired.